The proposed research involves the in vitro culture of primary fetal cells of extra-renal origin responsible for the monitoring and biosynthesis of erythropoietin. Such cultures would be established in a controlled environmental steady-state system which would permit investigation of the effects of variation in gas tensions, pH and substrates on the regulation of erythropoietin biogenesis while maintaining other environmental gases and factors at a constant level. Identification of cell types responsible for erythropoietin synthesis via immunofluorescence and concentration of these cells through cell separation techniques would lead to a comprehension of the mechanism by which an erythropoietin generating cell monitors and controls erythropoiesis. These studies should allow for a better understanding of the control of erythropoietin production in man, and be of practical use in the study of erythropoietin disturbed states.